The invention concerns a process for purification of an acidic monomer having a double bond, a device for synthesis of an acidic monomer having a double bond, a process for producing an acidic monomer having a double bond, an acidic monomer having a double bond and obtainable by this process, fibres, formed bodies, films, foams, superabsorbent polymers and other special polymers based on or containing this acidic monomer, the use of this acidic monomer in or for producing fibres, formed bodies, films, foams, superabsorbent polymers or other special polymers.
Due to the fact that mass produced synthetic materials are today increasingly used in areas in which high demands are made concerning hygiene and purity grade of the synthetic material, the provision of pure monomers for large-scale synthesis of synthetic materials becomes ever more important.
These demands on monomer purity, which are coming increasingly to the fore, correspond to monomer syntheses that are accordingly improved and lead to greater purity in ever-improved reactors and with new catalyst systems.
Despite the significant efforts in the area of reactor and catalyst research the crude monomer streams coming out of the reactors are still so impure, that these crude monomer streams must be subjected to a careful purification, before the thus obtained pure monomer can be worked into correspondingly pure synthetic materials by polymerization. The purification of the crude monomer stream in the technical production of monomers therefore takes on considerable importance.
Another parameter which is gaining increasing importance in the further development of technical syntheses of monomers and their treatment is the environmental aspect. In this context the use of water or aqueous systems is accorded great importance. Furthermore, the avoidance of waste as well as the achievement of higher yields with high conversions contributes to the environmental friendliness of technical syntheses. It is, therefore, important in technical syntheses of monomers to also recover and reuse quantities of monomer which seem small as a percentage proportion, which can form in many different flow-offs of the technical monomer synthesis, in order to optimise in this way the yield of the technical monomer synthesis.
An example of the currently described trend is the synthesis of water-absorbing polymers, which are used in hygiene articles such as diapers, sanitary napkins or incontinence articles. Very high demands on purity are made on these water-absorbing polymers. The demands on purity of the acrylic acids used in the production of these water absorbing polymers are therefore correspondingly high. Another example of technically produced polymers for which high demands on purity are made is the area of polymers used in treatment of drinking water. In this regard, non-cross-linked, linear polyacrylic acids are often concerned, which, by reason of the high demands on purity, are produced from acrylic acid which is as pure as possible.
U.S. Pat. No. 4,720,577 teaches the purification of low molecular carboxylic acids having no double bond, such as acetic acid, by use of an extraction auxiliary formed from aliphatic amines and phenol. The use of an extraction auxiliary comprising two components, the amine and the phenol, is complicated in comparison with a single component extraction auxiliary.
U.S. Pat. No. 3,997,599 discloses the purification of carboxylic acids such as methacrylic acid as aqueous phase by extraction with trioctylphosphine oxide in an organic solvent such as kerosene. The use of an organic solvent which is on the one hand environmentally polluting and is furthermore easily combustible is a disadvantage.
DE 21 36 396 teaches a purification of acrylic acid by counter-current washing with an extremely hydrophobic solvent, wherein organic solvents are preferably used as solvent, which carry an increased fire risk and environmental pollution.
DE 863 050 describes the purification of aqueous acrylic and methacrylic acids by extraction via addition of concentrated sulphuric acid, wherein an upper acrylic and methacrylic acid-rich phase forms. In this phase, however, a number of impurities such as benzaldehyde, propionic acid or dimeric acrylic and methacrylic acids also accumulates compared to the starting mixture used. This is disadvantageous for the large-scale application of this extraction.
In U.S. Pat. No. 3,663,375 discloses a three-step process for purification of a mixture containing isobutyric acid and methacrylic acid by salting out with sulphuric acid or sodium sulphate, wherein in a first step this mixture is salted out with formation of an aqueous and an organic phase, in a second step the phases are separated from each other and in a third step the separated phases respectively separated from each other are worked up by distillation. This teaching is disadvantageous on the one hand because of the many steps and on the other because of the thermally cumbersome distillation of monomers such as acrylic and methacrylic acids. During the distillation of acrylic and methacrylic acids dimers or oligomers of acrylic and methacrylic acids are particularly easily formed, which are undesirable in the large scale further processing into polymers.
DE 196 06 877 A1 discloses the extraction of an acidulous water side-stream with a small partial current of an almost acrylic acid-free solvent. The main stream of the acrylic acid gas stream obtained by gas phase oxidation is first worked up by absorption with a high boiling point solvent, followed by distillation and crystallization. The extraction disclosed here is disadvantageous through the use of the almost acrylic acid-free solvent and is not suitable for the main stream.
The general object of the present invention is, on the one hand, to overcome the disadvantages arising from the prior art.
On the other hand, an object according to the invention is to achieve, as inexpensively and environmentally friendly as possible and with a high yield, the extraction of the synthesised monomers from a mixture which contains further impurities in addition to the desired synthesised monomers.
Furthermore, it is an object of the invention consists to enable the purification of monomers with a phase former composed of as few components as possible.
An additional object according to the invention is to achieve purification of a monomer in as few steps as possible.
A further object according to the invention is, during purification of monomers, to deplete the various accompanying impurities simultaneously by a cleaning procedure, if possible.
A further object according to the invention lies in the provision of as gentle a monomer purification as possible, since the monomers are mostly reactive compounds and should be thermally stressed as little as possible.